Multiple medical disorders are linked to dysfunction of one or more neurotransmitter (e.g., dopamine, norepinephrine, and acetylcholine) systems. One class of such medical disorders is neurological disorders, such as Parkinson's disease, attention deficit hyperactivity disorder, restless legs syndrome, and sexual dysfunction, where dysfunction of the dopamine and/or norepinephrine systems have been shown to play a significant role. Dysfunction of the dopamine system has also been linked to drug addiction. Drugs such as cocaine and amphetamine have been reported to amplify the effects of dopamine. The dopamine system also has an impact on patients' cognitive function. Too little dopamine or too much dopamine has been reported to impair cognitive function, such as working memory and attention.
Therapeutics have been commercialized for treating disorders associated with dysfunction of neurotransmitter systems. One such example is bupropion hydrochloride, which has been approved by the United States Food and Drug Administration for treatment of depression, seasonal affective disorder, and as an aid for smoking cessation. Bupropion hydrochloride is marketed under the registered trademark WELLBUTRIN® and Zyban® and the prescribing information for WELLBUTRIN® explains that bupropion is an inhibitor of neuronal uptake of norepinephrine and dopamine. It is also reported that bupropion is a functional antagonist of nicotinic acetylcholine receptor subtypes such as α3, α4β3, α3β4, and al and that inhibition at these receptors may affect bupropion pharmacological activity. The commercialized form of bupropion hydrochloride is a racemic mixture and multiple dose-dependent adverse side effects have been reported in patients receiving this therapeutic. Exemplary side effects include, for example, seizures, agitation, dry mouth, insomnia, headache, migraine, nausea, vomiting, constipation, and tremor.
Due to the increasing number of patients suffering from neurological and other disorders associated with dysfunction of neurotransmitter systems, and the limitations of existing therapies, such as adverse side effects, there is a need for new therapeutic agents for treating medical disorders associated with such dysfunction. Attention deficit hyperactivity disorder, for example, has been reported to be one of the most common childhood disorders, and reports indicate that the number of children being diagnosed with attention deficit hyperactivity disorder is increasing. Seasonal affective disorder is another neurological disorder in need of improved therapy. Seasonal affective disorder often causes patients to experience feelings of hopelessness, increased appetite leading to weight gain, loss of energy, inability to concentrate, loss of interest in work or other activities, sluggish movements, social withdrawal, unhappiness, and/or irritability. The present invention addresses these needs and provides other related advantages.